Femoral sizing instrument

ABSTRACT

A femoral sizing instrument for use in a knee joint replacement procedure has a housing which includes formations by which it can be located relative to the femoral posterior condyles. A cortex arm has a tip for engaging the anterior cortex of the femur and a sulcus arm has a tip for engaging the sulcus. Each of the cortex and sulcus arms can be adjusted relative to the housing to adjust the distance between its tip and the posterior condyles measured generally parallel to the anterior posterior axis. The instrument includes at least one scale for indicating the positions of the tips of the cortex and sulcus arms relative to the posterior condyles.

This invention relates to a femoral sizing instrument for use in a kneejoint replacement procedure.

Femoral components of knee joint prostheses are generally available inseveral sizes to accommodate variations in bone size and shapes betweenpatients. Preparing the distal femur to receive the femoral component ofa knee joint prosthesis involves resecting the bone so that it isappropriately shaped to ensure that the femoral component is anappropriately close fit. Commonly used surgical techniques involvemaking distal, anterior, posterior, anterior chamfer and posteriorchamfer cuts. The locations of the cuts are determined according to thesize of the selected implant.

It is common to choose the size of the femoral component with referenceto the measured distance along the anterior posterior axis between theposterior condyle and the anterior cortex. This can enable a close fitbetween the anterior flange of the femoral component and the anteriorcortex.

The present invention provides an instrument which can be used to choosethe size of a femoral component with reference to the measured distancebetween the anterior and posterior condyles.

In one aspect, the invention provides a femoral sizing instrument foruse in a knee joint replacement procedure, which comprises:

-   -   a. a housing which includes formations by which it can be        located relative to the femoral posterior condyles,    -   b. a cortex arm having a tip for engaging the anterior cortex of        the femur,    -   c. a sulcus arm having a tip for engaging the sulcus, in which        each of the cortex and sulcus arms can be adjusted relative to        the housing to adjust the distance between its tip and the        posterior condyles measured generally parallel to the anterior        posterior axis, and    -   d. at least one scale for indicating the positions of the tips        of the cortex and sulcus arms relative to the posterior        condyles.

The instrument of the invention has the advantage that it enables thesize of a patient's femur, measured between the anterior and posteriorcondyles, to be taken into account when selecting an appropriate size offemoral component of a knee joint prosthesis. These data can be takeninto account together with data from the measurement of the distancebetween the posterior condyles and the anterior cortex. Factors whichcan affect the choice of the size of the femoral component include:

The femoral component should be sufficiently large to avoid the need tocreate a notch in the anterior cortex at the proximal edge of thecomponent. Such a notch can be a point of weakness in the bone afterimplantation of the prosthesis.

The femoral component should not be so large that it leads to anundesirable discontinuity between the anterior flange of the componentat its proximal edge between it and the anterior cortex.

The femoral component should be sufficiently large that soft tissueconnected to the patella is maintained under sufficient tension duringflexion to ensure that the joint between the patella and the implantedfemoral component is stable.

The femoral component should not be so large that tension on soft tissueconnected to the patella restricts flexion.

These fitting requirements can be addressed alternatively by appropriateadjustment of the position of the femoral component along the anteriorposterior axis to suit a patient in whose knee the sulcus depth which isgreater or less than would be suitable to accommodate a particularfemoral implant component. It can be seen accordingly that themeasurement techniques that are provided by the present invention enableselection of either or both of implant size and implant position to beoptimised.

The cortex arm can be used to measure the patient's femur along theanterior posterior axis, between the posterior condyles and the anteriorcortex. This measurement can be performed by placing the housingappropriately relative to the femoral posterior condyles, and moving thecortex arm until its tip is in contact with the anterior cortex. As isknown, for example in the instrument system which is sold by DePuyOrthopaedics Inc under the trade mark Sigma High Performance Classic, ascale can be included which can provide a surgeon with an indication ofthe appropriate size of femoral component which should be used. Theappropriate size of femoral component can help to minimise anydiscontinuity between the anterior flange of the component at itsproximal edge between it and the anterior cortex, while also minimisingthe need to create a notch in the anterior cortex at the proximal edgeof the component. Such a notch can be a point of weakness in the boneafter implantation of the prosthesis.

The sulcus arm can be used to measure the patient's femur along theanterior posterior axis, between the posterior condyles and the sulcus(being the shallow valley between the anterior condyles). Thismeasurement can be performed by placing the housing appropriatelyrelative to the femoral posterior condyles, and moving the sulcus armuntil its tip is in contact with the sulcus. The measurement willgenerally be taken at the point along the valley between the anteriorcondyles at which the distance from the posterior condyle to the valley(measured parallel to the anterior/posterior axis) is at a maximum.Information from this measurement can indicate to the surgeon theappropriate size of femoral component which should be used so that softtissue connected to the patella is maintained under sufficient tensionduring flexion to ensure that the joint between the patella and theimplanted femoral component is stable, but not so large that tension onsoft tissue connected to the patella restricts flexion.

The cortex and sulcus arms should be capable of movement relative to theposterior condyles so that they can provide independent measurement datafor the two measurements (between the posterior condyles and theanterior cortex and the sulcus respectively). A first one of the cortexand sulcus arms can be mounted on the housing so that the height of thesaid first arm along the anterior posterior axis relative to the housingcan be adjusted. The second one of the cortex and sulcus arms can bemounted on the first arm so that the height of the said second arm alongthe anterior posterior axis relative to the housing can be adjusted. Forexample, the cortex arm can be mounted on the housing so that its heightrelative to the housing can be adjusted. The sulcus arm can be mountedon the cortex arm so that it moves relative to the housing with thecortex arm. The sulcus arm can be moved relative to the cortex arm. Inuse, the housing is located relative to the posterior condyles and theanterior arm is moved until its tip is in contact with the anteriorcortex. The sulcus arm is then moved relative to the cortex arm untilits tip is in contact with the sulcus.

Alternatively, the sulcus arm can be mounted on the housing so that itsheight relative to the housing can be adjusted. The cortex arm can bemounted on the sulcus arm so that it moves relative to the housing withthe sulcus arm. The cortex arm can be moved relative to the sulcus arm.In use, the housing is located relative to the posterior condyles andthe sulcus arm is moved until its tip is in contact with the sulcus. Thecortex arm is then moved relative to the sulcus arm until its tip is incontact with the anterior cortex.

When a first one of the cortex and sulcus arms is mounted on the secondone of the cortex and sulcus arms, so that the first arm can slide alongthe second arm, the nature of the engagement between the first andsecond arms can be such that the path which is defined for the secondarm to slide along is not parallel to the axis of the bone (when thebone is viewed from one side along the lateral-medial axis). Forexample, the second arm might define a ramp for the first arm to slidealong. The ramp surface might be inclined so that the first arm isdisplaced in an anterior direction away from the bone axis as it is slidalong the second arm in a superior to inferior direction. The rampsurface might be inclined so that the first arm is displaced in aposterior direction towards the bone axis as the arm is slid along thesecond arm in an inferior to superior direction. The ramp might arise asa result of the second arm being formed with an appropriate shape. Forexample, the ramp might arise as a result of a variation in thethickness of the material of the second arm, or by bending an arm madewith a constant thickness.

The selection of a ramp shape can be selected according to knownvariations across the relevant population in the distance from theposterior condyle to the sulcus relative to the distance from theposterior condyle to the anterior cortex, and the correspondingdistances across a range of femoral components which are designed toaccommodate those variations across the population. The ramp willfrequently have an approximately constant inclination along its length.The ratio of the anterior/posterior height to the inferior/superiorlength will generally be not more than about 1.0, preferably not morethan about 0.9, for example about 0.8. The said ratio will generally beat least about 0.5, preferably at least about 0.6.

For example, in an embodiment of an instrument according to theinvention, the sulcus arm can be mounted on the cortex arm so that thesulcus arm can slide along the cortex arm. The surface of the cortex armwhich faces away from the bone, against which the sulcus arm acts, canbe inclined to the bone axis so that the distance between that surfaceand the axis increases in the superior to inferior direction. This canbe achieved by increasing the thickness of the cortex arm.Alternatively, the distance between the surface of the cortex arm andthe bone axis might decrease in the inferior to superior direction.Furthermore, the movement of a first arm along a second arm might be ina direction which is approximately parallel to the bone axis when theviewed is viewed from one side along the lateral-medial axis.

The relationship between the path followed by one of the arms relativeto the bone axis (as discussed above) can be determined to take accountof the configuration of the implant component with which the instrumentis to be used and the variation in dimensions of the implant componentbetween sizes. In particular, a ramp can give rise to advantages whenthe thickness of the superior portion of a femoral component variesbetween different sizes of the component.

The instrument can include a first scale for indicating the distancebetween the tip of the cortex arm and the posterior condyles, and asecond scale for indicating the distance between the tip of the sulcusarm and the posterior condyles. The instrument can include a first scalefor indicating the distance between the tip of a first one of the cortexand sulcus arms and the posterior condyles, and a second scale forindicating the distance between the tips of the sulcus and condyle arms.The distance between the tip of the second one of the cortex and sulcusarms and the posterior condyles can be derived from the second scale.Use of a second scale for indicating the distance between the tips ofthe sulcus and condyle arms will generally involve making an initialassessment of the appropriate size of a femoral implant component basedon the measured distance between the tip of one of the arms and theposterior condyles, with the measurement of the distance between thetips of the two arms being used to assess whether a variation in theinitial assessment is required. For example, a first scale can be usedto indicate the distance between the posterior condyles and the anteriorcortex. A second scale can be used to determine whether the distancebetween the anterior cortex and the sulcus is such that a variation onthe size of implant which is determined using the first scale isappropriate in order to ensure appropriate tension in soft tissueassociated with the patello-femoral joint.

The scale which is provided to indicate a measurement can provideinformation concerning the absolute distance from the posterior condylesto the anterior cortex or to the sulcus as the case may be. It willhowever frequently be preferred for the scale to provide informationdirectly concerning the size of the femoral component of a knee jointprosthesis which is suitable according to the measured distance.

When a scale is provided to provide information concerning distancebetween the tips of the sulcus and condyle arms, it can provideinformation concerning the absolute distance between the tips. It willhowever frequently be preferred for the scale to provide informationconcerning the variation from the statistically derived expecteddistance, which might cause a surgeon to vary the size of the femoralcomponent which is to be implanted. For example, the scale mightindicate that the size of femoral component should be reduced by onesize, or that it should be increased by one size. For example, one ofthe sulcus and condyle arms might provide an indication of the implantsize, and the other of the arms might provide an indication that thesize should be increased or decreased by one size.

The arm or each arm which is mounted on the housing should preferably becapable of adjustment so that the distance between the posteriorcondyles and the tip of the arm, measured generally parallel to theanterior posterior axis, can be adjusted, and also so that the effectivelength of the arm, measured between the housing and the tip measuredgenerally along the axis of the bone, can be adjusted.

Preferably, the cortex arm is capable of adjustment so that the distancebetween the posterior condyles and its tip, measured generally parallelto the anterior posterior axis, can be varied through at least about 40mm. Preferably, the distance will be variable through not more thanabout 75 mm.

Preferably, the sulcus arm is capable of adjustment so that the distancebetween the posterior condyles and its tip, measured generally parallelto the anterior posterior axis, can be varied through at least about 40mm. Preferably, the distance will be variable through not more thanabout 80 mm.

Preferably, the cortex and sulcus arms are capable of adjustment so thatthe distance between their tips, measured generally parallel to theanterior posterior axis, can be varied through at least about 5 mm.Preferably, the distance will be variable through not more than about 15mm.

Preferably, the effective length of the cortex arm along the superiorinferior axis measured between the housing and the tip of the cortex armcan be adjusted. Preferably, the effective length of the sulcus armalong the superior inferior axis measured between the housing and thetip of the sulcus arm can be adjusted.

It can be preferred that the first one of the cortex and sulcus armsdefines a track for the second one of the cortex and sulcus arms toslide along to adjust the distance between the tips of the cortex andsulcus arms, measured along the superior inferior axis, and in which theinstrument includes a locking feature to lock the second one of thecortex and sulcus arms against movement along the track. For example,one of the arms can have a slot formed in it, and the other arm can havea spigot which extends through the slot, and which can slide along theslot. Such a spigot can be threaded at its free end, to receive a nutwhich can clamp against a top face of the slot to provide the lock.Other track arrangements can be envisaged, for example in which thesecond arm has a groove formed in it at one end, which can receive thefirst arm.

It can be preferred for the housing to have a first location face whichcan be located relative to the distal face of the femur, and a secondlocation face which can be located relative to a posterior condyle. Thiscombination of first and second location faces can be used to locate thehousing relative to the posterior condyles of the femur while performingthe anterior posterior measurements using the instrument. As is commonlythe case with existing instruments, the measurements of the anteriorposterior dimension of the femur will generally be performed with theinstrument of the invention after an initial distal resection. Oneexample of a technique for determining the appropriate location andorientation of the distal resection plane uses the plane on which thetibia has been resected as a point of reference. The first location faceof the housing component of the instrument of the invention can then beplaced in contact with the resected distal surface of the femur.

Preferably, the housing includes a connector leg which extends betweenthe housing and one of the cortex and sulcus arms and is movablerelative to the housing so that the length of the connector leg,measured from the housing to the said one of the cortex and sulcus arms,can be adjusted. This can provide for adjustment of the distancemeasured generally parallel to the anterior posterior axis between theposterior condyles and the tip of one or each of the arms. Theinstrument can include a leg locking device by which the position of theconnector leg relative to the housing can be locked.

In some constructions, the extension of the connector leg relative tothe housing might be described as telescopic.

The connector leg can be received in a slot in the housing, in which itcan slide. The connector leg might have a groove formed in it, in whicha rib on the housing can be received. The connector leg and the housingcan provide a scale which indicates the extent of the extension of theconnector leg relative to the housing, and therefore of the distancemeasured generally along the anterior posterior axis from the posteriorcondyles to the tip of the arm which is connected to the housing by theconnector leg. This distance can be related to the size of theappropriate implant which should be used according to the size of thepatient's femur.

Preferably, the cortex arm or the sulcus arm which is connected to thehousing by means of the connector leg can slide relative to theconnector leg. This can provide for adjustment of the effective lengthof the arm, measured between the housing and the tip of the arm.

The instrument can include an arm locking device to lock the arm againstsliding relative to the connector leg. Locking devices which can be usedto lock an arm against sliding relative to a connector leg are known inexisting measurement devices such as the measuring device in theinstrument system which is sold by DePuy Orthopaedics Inc under thetrade mark Sigma High Performance Classic. A suitable locking device caninclude a threaded nut.

The housing can include formations such as hollow bores by which it canbe fastened to a femur, for example using pins or screws, if required bya surgeon.

In another aspect, the invention provides a method of measuring thedistal femur in a knee replacement procedure using an instrumentaccording to the invention, which comprises:

-   -   a. locating the housing relative to the femoral posterior        condyles,    -   b. moving a first one of the sulcus and cortex arms so that its        tip is in contact with the sulcus or the anterior cortex        respectively,    -   c. deriving information as to the size of the femur from the        position of the tip of the first one of the sulcus and cortex        arms relative to the femoral posterior condyles to identify an        implant component provisionally,    -   d. moving a second one of the sulcus and cortex arms so that its        tip is in contact with the sulcus or the anterior cortex        respectively,    -   e. assessing based on information as to the distance between the        tip of the second one of the sulcus and cortex arms relative to        the femoral posterior condyles whether to vary the size and or        the anterior/posterior position of the implant component        identified in step (c).

The instrument of the invention can be made from materials which areconventionally used in the manufacture. These frequently include metalssuch as certain stainless steels. Polymeric materials can be used forall or part of the instrument.

The instrument of the invention can be used to determine the size of thefemur for the purpose of implanting a femoral component of a whole kneejoint prosthesis.

The instrument of the invention can be used to determine the size of thefemur for the purpose of implanting a femoral component of a partialknee joint prosthesis, for example in which just one compartment of thejoint is replaced (which might be the medial compartment or the lateralcompartment).

The instrument of the invention can be used to determine the size of thefemur for the purpose of implanting a partial femoral component, forexample in which the component is a patellofemoral component which canbe fitted to the anterior face of the femur.

Embodiments of the invention will now be described by way of examplewith reference to the accompanying drawings, in which:

FIG. 1 is an isometric view of a femoral sizing instrument for use in aknee joint replacement procedure.

FIG. 2 is a view along the medial lateral axis of a knee joint afterimplantation of a knee joint prosthesis.

FIG. 3 is a view along the medial lateral axis of the instrument shownin FIG. 1, in position relative to the distal end of a patient's femur.

FIG. 4 is an isometric view of another embodiment of a femoral sizinginstrument for use in a knee joint replacement procedure.

Referring to the drawings, FIG. 1 shows an instrument 2 for measuring afemur in a knee joint replacement procedure. It comprises a housing withposterior plates 4 and a distal plate 6. The housing has a bore 8 formedin it in which a connector leg 10 can be received and can slide, in adirection which is generally parallel to the distal plate 6. Theposition of the connector leg can be clamped against sliding relative tothe housing by means of a threaded screw which can be screwed into athreaded hole in the housing so that the end of the screw acts againstthe connector leg. Locking screws of this type are known.

The connector leg has a hub 12 its upper end with a bore extendingthrough it. The connector leg has a scale 13 marked on it.

The instrument includes an anterior arm 14. The anterior arm is sized tobe a sliding fit in the bore in the hub 12 on the connector leg. Theanterior arm 14 has a tip 16. It has a slot 18 formed in it extendingalong part of its length. The thickness of the anterior arm increasesalong the length of the slot, towards the tip of the arm so that theanterior face 19 of the arm defines an inclined ramp surface.

The instrument includes a sulcus arm 20. The sulcus arm is mounted in aplate 22 which fits on to the anterior arm and can be slid along theslot 18 therein. The anterior arm has first a scale 23 on one side toindicate the position of the sulcus arm 20 and of its plate 22 along thelength of the slot. It has a second scale 27 on it to indicate theposition of the anterior arm relative to the hub 12 on the connectorleg.

The sulcus arm can slide within the plate 22. The sulcus arm has a tip24. The sulcus arm has a scale 25 on one side to indicate the positionof the sulcus arm within the plate.

FIG. 2 shows a knee joint between a femur 30 and a tibia 32. A tibialcomponent 34 of a knee joint prosthesis has been implanted in the tibia.A femoral component 36 has been implanted in the femur. The patella 38articulates against the femoral component during flexing of the joint.

The present invention is concerned with determining the size of thefemur so that an appropriately sized femoral component 36 of the kneejoint prosthesis is selected for implantation. The component should besufficiently large to avoid the need to create a notch in the anteriorcortex 40 at the proximal edge 42 of the component. Such a notch can bea point of weakness in the bone after implantation of the prosthesis.However, the component should not be so large that it leads to anundesirable discontinuity between the anterior flange of the componentat its proximal edge 42 between it and the anterior cortex 40. This isdetermined with reference to the distance measured parallel to theanterior posterior axis between the posterior condyles and the anteriorcortex in the patient's natural knee (as described below with referenceto FIG. 3)

The femoral component 36 should be sufficiently large that soft tissueconnected to the patella 38 is maintained under sufficient tensionduring flexion to ensure that the joint between the patella and theimplanted femoral component is stable. However, the component should notbe so large that tension on soft tissue connected to the patellarestricts flexion. This is determined with reference to the distancemeasured parallel to the anterior posterior axis between the posteriorcondyles and the sulcus in the patient's natural knee (as describedbelow with reference to FIG. 3)

FIG. 3 shows the instrument of the invention positioned against apatient's femur during preparation of the femur to receive the femoralcomponent of a knee joint prosthesis. As shown in FIG. 3, the femur hasbeen resected distally to create a planar distal face 30. The positionand orientation of the distal resection plane can be determined usingconventional techniques.

The size of the femur is determined for the purpose of selecting anappropriately sized femoral component. A first measurement which is madeconventionally between the patient's posterior condyles 50 and theanterior cortex 52, measured parallel to the anterior posterior axis 54.This is to minimise the risks of (a) having to create a notch in theanterior cortex at the proximal edge of the selected femoral component,and (b) an undesirable discontinuity between the anterior flange of thecomponent at its proximal edge between it and the anterior cortex.

A second measurement is between the patient's posterior condyles 50 andthe sulcus (that is, the shallow valley between the anterior condyles),measured parallel to the anterior posterior axis 54. This is to minimisethe risk of soft tissue connected to the patella being in appropriatelytensioned so that the patello-femoral joint is not stable or so thatflexing of the knee joint after joint replacement is unduly restricted.

Accordingly, in a first measurement step, the instrument of theinvention is placed with the distal plate 6 in contact with the resecteddistal face of the femur and the posterior plates 4 in contact with theposterior condyles 50. The connector leg 10 is slid out of the bore 8 inthe housing sufficiently to allow the anterior arm 14 to be movedparallel to the superior inferior axis of the femur. The anterior arm isthen moved until its tip 16 is positioned beyond the interface 56between the anterior condyles and the anterior cortex (which is thepoint to which the anterior flange of the femoral component of the kneejoint prosthesis will extend once implanted). The connector leg is thenslid into the bore 8 in the housing until the tip of the anterior armcontacts the femur. Throughout this manipulation of the anterior arm 14,the sulcus arm 20 is maintained in a retracted position relative to theplate 22 so that its tip 24 is spaced apart from the anterior face ofthe femur.

The scale 13 on the connector leg provides a first indication of thesize of the patient's femur. This provides a point of reference forsubsequent measurement steps. The scale can provide an indication of theabsolute distance (for example in millimetres) between the posteriorcondyles and the anterior cortex, measured generally parallel to theanterior posterior axis. Alternatively, it can be preferred for thescale to provide directly an indication of the size of a femoralcomponent which is suitable for the size of the measured femur (forexample ranging in integers from 1 to 7).

Accordingly, the first size indication from the scale 13 on theconnector leg is used to set the position of the anterior arm 14relative to the hub 12, using the scale 27 on the arm.

The second measurement step is performed once the tip 16 of the anteriorarm 14 has been positioned in contact with the anterior cortex. Thesulcus arm 20 with its plate 22 is moved along the anterior arm 14 inthe slot 18, and positioned relative to the anterior arm using the firstindication which is derived from the scale 13 on the connector leg (asdescribed above). This movement will be along the ramp surface. Thesulcus arm is then lowered through the plate until its tip 24 is contactwith the sulcus, measured at the point along the valley between theanterior condyles at which the distance from the posterior condyle tothe valley (measured parallel to the anterior/posterior axis) is at amaximum.

It will be noted that the position of the anterior arm relative to thehub (indicated by scale 27 on the anterior arm to indicate the positionof the arm relative to the hub), and the position of the sulcus armwithin the slot in the anterior arm (indicated by the scale 23 on theanterior arm for the sulcus arm), are both set with reference to thefirst size indication from the scale 13. The markings on the hub scale27 are derived from the distance from the distal condyle to the anteriorflange across the range of sizes of the femoral implant component. Themarkings on the sulcus arm scale 23 are derived from the maximumthickness of the anterior flange across the range of sizes of thefemoral implant component. Accordingly, the distance between the sizemarkings on the hub scale 27 is greater than the distance between themarkings on the sulcus arm scale 23. For example, in a preferredembodiment, the distance between the markings on the hub scale 27between the smallest and largest sizes of implant in a size range mightbe 15 mm and the distance between the markings on the sulcus arm scale23 between the largest and smallest sizes of implant in that size rangemight be 5 mm.

The scale 25 on the sulcus arm 20 can provide directly an indication ofthe suitability of the size of the femoral component as determined inthe first measurement step, having regard to the measured distancebetween the posterior condyles and the sulcus. Accordingly, the scale 25might provide a set of readings ranging from “−2”, through “0”, to “+2”.On this scale, a reading of “0” from the measurement of the posteriorcondyle to sulcus distance would indicate that the size of the femoralcomponent derived from the first size measurement is appropriate tomaintain appropriate soft tissue tension in the patello-femoral joint. Areading which is less than “0” would indicate that a smaller size offemoral component should be selected in order to avoid a restriction onjoint flexion as a result of soft tissue in the patello-femoral jointbeing too tight. A reading which is greater than “0” would indicate thata larger size of femoral component should be selected in order thatstability of the patello-femoral joint is not compromised. In order toprovide an indication of the suitability of the component size using ascale from, for example “−2” to “+2” when the thickness of the anteriorflange of the femoral component varies across the size range, it willgenerally be necessary for the instrument to include the ramp feature asdiscussed above, where the ramp is configured to take account of therange of thickness of the anterior flange, and the length from end ofthe anterior flange the point along its length at which the thickness ofthe flange is at a maximum. When the thickness of the anterior flangedoes not vary across the size range, the scale on the sulcus arm canprovide an indication of the suitability of the component size using ascale from, for example “−2” to “+2” without the ramp feature.

It is also possible to use the scale on the sulcus arm to provide anindication of the absolute distance (for example in millimetres) betweenthe tip 24 of the sulcus arm and the anterior arm. From this, it ispossible to calculate the distance between the tip and the posteriorcondyles, using the size information obtained in the first measurementstep described above. This can be used to identify an appropriate sizeof femoral implant component having regard to the distance between theposterior condyles and the sulcus.

FIG. 4 shows an instrument 200 for measuring a femur in a knee jointreplacement procedure. It comprises a housing with posterior plates 204and a distal plate 206. The housing has a bore 208 formed in it in whicha primary connector leg 210 can be received and can slide, in adirection which is generally parallel to the distal plate 206. Theposition of the primary connector leg can be clamped against slidingrelative to the housing by means of a threaded screw which can bescrewed into a threaded hole in the housing so that the end of the screwacts against the connector leg. Locking screws of this type are known.

The connector leg has a primary hub 212 its upper end with a boreextending through it. The connector leg has a scale 213 marked on it.

The instrument includes a sulcus arm 214. The sulcus arm is sized to bea sliding fit in the bore in the hub 212 on the connector leg. Thesulcus arm 214 has a tip 216. It has a slot 218 formed in it extendingalong part of its length.

The primary hub 212 and the primary connector arm 210 have a boreextending at least into the upper part of the connector arm.

The instrument includes a cortex arm 220 having a tip 221. The cortexarm is mounted on a secondary connector arm 222 which is a sliding fitin the bore in the primary hub 212 and the primary connector arm 210.The cortex arm is a sliding fit in a bore in a secondary hub 224 whichis provided on the secondary connector arm. A scale 226 is provided onthe secondary connector arm.

The height of the sulcus arm 214 relative to the posterior plates 204can be adjusted by sliding the primary connector arm 210 relative to thehousing.

The height of the cortex arm 220 relative to the sulcus arm and to theposterior plates can be adjusted by sliding the secondary connector arm222 relative to the primary connector arm 210.

Each of the sulcus arm and the connector arm can be slid in itsrespective hub relative to its connector arm so that the effectivelength of the arm is suitable to position the tip of the arm on thesulcus or on the anterior cortex. Each of the arms can be restrainedselectively against sliding relative to its hub by means of, forexample, a locking screw or a series of indents on the arm operating inconjunction with a resiliently deformable tooth.

The distance from the posterior condyle to the sulcus can be determinedby inspecting the scale on the primary connector arm. The distancebetween the sulcus and the anterior cortex, measured in a directionwhich is generally perpendicular to the axis of the femur, can bedetermined by inspecting the scale on the secondary connector arm. Thedistance from the posterior condyle to the anterior cortex can bedetermined by inspecting the scales on the primary and secondaryconnector arms. The distance information derived in this way can be usedto select a size of femoral component of a kneed joint prosthesisaccording to the measured size of the patient's femur.

1-14. (canceled)
 15. A femoral sizing instrument for use in a knee jointreplacement procedure, wherein the knee joint replacement procedureincludes replacing the distal femur, the distal femur having an anteriorcortex, medial and lateral condyles with distal and posterior surfacesand a sulcus, which comprises: a housing having formations by which thehousing can be located relative to the posterior surface of at least oneof the medial and lateral femoral condyles; a primary connector legextending outwardly from the housing; a primary hub attached to theprimary connector leg; a secondary connector leg extending outwardlyfrom the primary hub; a secondary hub attached to the secondaryconnector leg; a cortex arm having a first portion extending outwardlyfrom the secondary hub in one direction and a second portion having atip for engaging the anterior cortex of the femur, the second portionextending from the first portion in a second direction different fromthe first direction; a sulcus arm having a first portion extendingoutwardly from the primary hub in a third direction and a second portionhaving a tip for engaging the sulcus, the second portion extending fromthe first portion in a fourth direction different from the thirddirection; wherein: the cortex arm is slidable in the secondary hub inthe first direction and the sulcus arm is slidable in the primary hub inthe third direction; and the primary connector leg includes a scale andthe secondary connector leg includes a scale.
 16. The instrument ofclaim 15, wherein the primary connector leg is slidable with respect tothe housing so that the height of the cortex arm relative to the housingcan be adjusted and the secondary connector leg is slidable with respectto the first connector leg so that the height of the cortex arm relativeto the sulcus arm can be adjusted.
 17. The instrument of claim 15,wherein the housing has a first location face for contacting the distalface of the femur, and a second location face for contacting a posteriorsurface of a medial or lateral condyle.